In-depth understanding of JAVA core virtual machine technology

The Java Virtual Machine (JVM) is the key to the Java language's ability to "write once and run on multiple platforms". Java code is compiled into bytecode, and then the JVM interprets and executes the bytecode, which is cross-platform and ensures operational security and stability. Therefore, a deep understanding of the core technology of JVM is crucial for Java developers. This article will introduce the main components of JVM and its working principle in detail, and give specific Java code examples to help readers better understand.

Main components of JVM

JVM is mainly composed of the following components:

1. Class loader (ClassLoader)

ClassLoader is a very important component in JVM. An important component whose main job is to dynamically load bytecode files into memory at runtime and convert them into Java classes. ClassLoader is divided into three types: startup class loader, extension class loader and application class loader.

In the following code example, we define a Java class named com.example.Test and use ClassLoader to load it:

public class ClassLoaderDemo {
    public static void main(String[] args) {
        ClassLoaderDemo demo = new ClassLoaderDemo();
        ClassLoader classLoader = demo.getClass().getClassLoader();
        try {
            Class clazz = classLoader.loadClass("com.example.Test");
            System.out.println("Class " + clazz.getName() + " loaded by " + clazz.getClassLoader());
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

2. Runtime data area (Runtime Data Area)

JVM stores runtime data through the runtime data area. It is divided into several parts such as method area, heap, stack, program counter and local method stack.

• Method area: stores loaded class information, constants, static variables and compiler-generated code, etc.
• Heap: stores dynamically allocated object instances. Heap allocation and recycling are the core of Java's garbage collection mechanism.
• Stack: stores local variables, method parameters, operand stacks (temporary variables used when executing methods) of each thread, etc. Each method creates a stack frame when it is executed, and the stack frame is destroyed when the method ends.
• Program counter: records the address of the bytecode instruction being executed by the current thread.
• Native method stack: Similar to a stack, it is used to store information such as parameters and return values ​​when each thread calls a local (Native) method.

3. Bytecode execution engine (Execution Engine)

The bytecode execution engine is the core component of the JVM. It is responsible for interpreting and executing Java bytecode, and can also Bytecode is compiled into native machine instructions for execution. The bytecode execution engine usually uses an interpreter to execute the bytecode, but for frequently executed methods, it will use a just-in-time compiler (JIT) to compile the bytecode into local machine instructions to Improve program performance.

4. Garbage Collector

The Java garbage collection mechanism solves memory management problems by automatically detecting objects that are no longer used and recycling them. The JVM's garbage collector stores unused objects in the heap and periodically scans the objects in the heap to find out the no longer used objects and recycle them.

The following is a simple Java code example that demonstrates how to create a useless object and trigger the garbage collection mechanism:

public class GarbageCollectionDemo {
    public static void main(String[] args) {
        for (int i = 0; i < 10000; i++) {
            Object obj = new Object();
            // do something with obj
        }
        System.gc(); // explicitly trigger garbage collection
    }
}

How the JVM works

In Java When the application starts, the JVM will first load the Java class and interpret and execute the bytecode. When executing bytecode, the JVM interprets the bytecode line by line into machine instructions that the operating system can recognize and execute. The data required by the bytecode is stored in the runtime data area and memory is allocated and released in the heap. If local methods are used in the program, you also need to use the local method stack to call the local method.

The JVM automatically recycles no longer used objects and releases memory through the garbage collector. If there is insufficient memory, the JVM will throw an OutOfMemoryError exception. During the life cycle of the JVM, the JVM executes Java bytecode through the execution engine and loads other dependent classes through the class loader.

The following code demonstrates how the class loader works:

public class ClassLoaderDemo {
    public static void main(String[] args) {
        ClassLoaderDemo demo = new ClassLoaderDemo();
        ClassLoader classLoader = demo.getClass().getClassLoader();
        try {
            Class clazz = classLoader.loadClass("com.example.Test");
            System.out.println("Class " + clazz.getName() + " loaded by " + clazz.getClassLoader());
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

In this example, we loaded the Test class through ClassLoader. ClassLoader will first search for classes contained in the classpath, and if it cannot be found, it will delegate the search to the parent class loader. If all parent class loaders cannot find the class, the application class loader (Application Class Loader) will eventually load the class. Before loading, ClassLoader also verifies the bytecode to ensure its safety and correctness.

Summary

JVM plays a vital role in Java development. Its working principle determines that Java can run across platforms and ensures the security and stability of the program. JVM consists of components such as class loader, runtime data area, bytecode execution engine, and garbage collector. Each component has different roles and functions. Understanding these components is very important for Java developers, and specific code examples are needed to deepen the understanding of the JVM.

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